378 THE VOYAGE OF H.M.S. CHALLENGER. 



proportions, and is not serrate on the surface. That on the fronto -lateral angle of the 

 carapace is reduced to a mere point, while that above the hepatic region is short and 

 slender, and one of the marginal spines has disappeared. The pleon has no dorsal teeth 

 anterior to the fifth and sixth somites, on each of which there exists a small tooth at the 

 posterior extremity. The teeth on the lateral margins exist as sharp points that arc 

 smooth and free from ornamentation. 



Observations. — This specimen appears to show the gradual transition of the form 

 Acanthosoma into that of Mastigopus, by the loss of the ecphyses and the gradual 

 formation of the pereiopoda, jKn-i passu with the reduction of the spinous character of 

 the animal. 



Mastigopus suhmi, n. sp. (PI. LXVI. fig. 2). 



Carapace, excluding the rostrum, one-third of the length of the body of the animal. 

 Eostrum less than half the length of the carapace, subapically armed with two or three 

 small teeth ; a small tooth stands on the outer angle of the orbit and another at the 

 antero-lateral angle of the carapace ; the lateral and posterior margins are smooth and 

 free from ornamentation. 



Pleon dorsally and laterally free from teeth or spines, except a small tooth on the 

 posterior dorsal margin of the fifth and sixth somites. 



Telson less than half the length of the sixth somite, broad at the base and suddenly 

 narrowing towards the extremity, where it is minutely forked. 



The ophthalmopoda are pyriform, narrow at the base and broad at the ophthalmus, 

 but the widening is not very sudden. 



The first pair of antenna? has the peduncle longer than the rostrum, and is three- 

 jointed ; the first joint is broad at the base, being expanded to contain the otocyst, and 

 the outer angle is armed with a tooth, from which the joint suddenly narrows and reaches 

 to about two-thirds of the length of the rostrum. The second joint is narrow, with 

 parallel margins, and reaches to the extremity of the rostrum ; the third joint is shorter 

 than the second and supports a long and a short flagellum (the longer flagellum is 

 broken off). 



The second pair of antennae has a broad coxal joint, which supports a flagellum that is 

 broken off and a scaphocerite that is as long as the rostrum, armed with a tooth on the 

 smooth side near the distal extremity, and furnished on the opposite and distal margins 

 with long and slender hairs. 



The oral appendages have not been closely examined. 



The first pair of gnathopoda has assumed its appearance in the adult, the ante- 

 penultimate joint having a knee-like bend near the meral articulation. 



The second pair of gnathopoda has the coxal joint short and broad, the second long 



